1. Field of the Invention
This invention relates to mixed protonic-electronic conducting material useful as H.sub.2 permeation membrane material or electrode material.
2. Prior Art
Compressed natural gas (CNG) is an economically competitive, widely distributed energy and chemical resource. The natural gas is converted to hydrogen more easily and efficiently than are liquid hydrocarbons and is less expensive per mole H.sub.2 produced than any other fuel. Systems for the production of hydrogen from natural gas could be enhanced through the use of thermally efficient, compact, catalytic membrane reactors.
Advances in membrane reactor technology allow economic production of high purity hydrogen from natural gas by coupling steam reforming and hydrogen transport in one step. Removal of product hydrogen continuously through the membrane shifts the equilibrium toward increased hydrogen production. Although palladium metal alloy membranes have been available for several decades, they are expensive and require large areas for adequate fluxes in commercial applications.
Recently, a series of perovskite-type oxides (e.g. BaCe.sub.1-x M.sub.x O.sub.3, where M is a metal dopant) have been shown to have a high proton conductivity at elevated temperature. These mixed ionic conductors are receiving considerable attention because of their numerous applications as electrolytes in fuel cells, hydrogen pumps, electrolyzers, and gas sensors, and are described for instance in Taniguchi et al. U.S. Pat. No. 5,387,330.
With the above perovskite-type oxides protonic conductivities have been measured on the order of 10.sup.-2 .OMEGA..sup.-1 cm.sup.-1 at 600.degree. C. This ionic conductivity is comparable to that observed for oxygen-ion conduction in La.sub.1-y Sr.sub.y Co.sub.1-x M.sub.x O.sub.3 perovskite-type oxides. La.sub.1-y Sr.sub.y Co.sub.1-x M.sub.x O.sub.3 oxides are mixed conductors in that they conduct both oxygen ions and electrons, and they have received considerable attention for application as oxygen permeation membranes. Because of their significant electronic conductivity, they have an internal electrical short and O.sub.2 will selectively permeate through the material under a differential oxygen partial pressure (P.sub.02). The potential permeation flux rates of these materials are extremely high. For example, calculations based on the results of Teraoka et al. "Influence of Constituent Metal Cations in Substituted LaCoO.sub.3 on Mixed Conductivity and oxygen Permeability," Solid State Ionics, 48 (1991) 207-212, show O.sub.2 flux rates through a 50-.mu.m-thick membrane of La.sub.0.6 Sr.sub.0.4 Co.sub.0.8 Cu.sub.0.2 O.sub.3 at 600.degree. C. to be 22400 L (STP) h.sup.-1.multidot.m.sup.-2 of membrane surface area under a 0.21 atm P.sub.02 gradient.
BaCe.sub.1-x M.sub.x O.sub.3 -type protonic conductors have sufficient ionic conductivity to obtain comparable flux rates. However, they have insufficient electronic conductivity. The electronic conductivity is necessary to balance the transport of charge through the material. If comparable electronic conduction could be obtained with the BaCe.sub.1-x M.sub.x O.sub.3 -type protonic conductors, they could be excellent H.sub.2 permeation membrane materials, equivalent to palladium alloy films.
A second potential application of solid-state high temperature protonic electrolytes is the production of higher hydrocarbons such as C.sub.6 H.sub.6 and C.sub.7 H.sub.8 from CH.sub.4 : EQU 6CH.sub.4.revreaction.C.sub.6 H.sub.6 +9H.sub.2
The decomposition and conversion of methane into benzene (C.sub.6 H.sub.6 and C.sub.7 H.sub.8) is thermodynamically favoured at moderate temperatures (500.degree. C.) and moderate pressures (1 to 10 atm) when hydrogen is continuously removed to low levels (&lt;0.05 atm). A suitable dehydrogenation catalyst with low coking tendency (Pt or Pd), combined with a small pore zeolite for hydrodecyclization of C.sub.2+ intermediates (such as C.sub.2 H.sub.4), could give high yields of aromatics. Electrochemical pumping, by application of a voltage across an H.sup.+ electrolyte is essential to increase the rate of H.sub.2 removal, since little driving force for H diffusion exists with low H.sub.2 partial pressures on both sides of the membrane.
Electrocatalytic conversion of methane to higher hydrocarbons and to syn gas has been reported in the literature. They are described in D.Eng and M. Stoukides, "The Catalytic and Electrocatalytic Coupling of Methane Over Yttria-Stabilized Zirconia," Catalysis Letters, 9 (1991) 47-54 and U. Balachandran et al., "Fabrication of Ceramic-Membrane Tubes for Direct Conversion of Natural Gas," Paper presented at the 1992 International Gas Research Conference. Both of these approaches used solid, oxygen-ion conducting ceramics. Under these conditions, both approaches are partial oxidation routes. At high conversions, partial oxidation runs the risk of producing undesirable, deep oxidation products (CO.sub.2 and H.sub.2 O), thus limiting H.sub.2 yield. A preferable route is to electrocatalytically abstract an H from CH.sub.4 by using a protonic conductor. The resulting CH.sub.3 fragments then form higher hydrocarbons in the reacting gas stream, and pure H.sub.2 is produced on the other side of the membrane.
For both of these applications, mixed protonic-electronic conducting materials are required. For electrocatalytic conversion the mixed conducting material is necessary for the electrodes, and for H.sub.2 permeation membranes it is the membrane material itself.
Galuszka et al. U.S. Pat. No. 5,637,259 describes a process for producing syn gas and hydrogen from natural gas using a membrane reactor. This used a hydrogen permeable membrane wall in the form of a porous alumina tube having a palladium film superimposed on the inner wall thereof.
A process for steam reforming of a hydrocarbon to produce H.sub.2, Co and CO.sub.2 is described in Minet et al. U.S. Pat. No. 4,981,676. That process also utilizes a hydrogen permeable membrane wall for separating hydrogen from the reaction zone.
It is an object of the present invention to provide a process for separating hydrogen from a hydrogen-containing gas by means of a membrane formed of a material that is both hydrogen ion conductive and electronic conductive.